1. Field of Invention
This invention relates to a semiconductor process, and more particularly to a method for improving the flatness of a layer deposited on a doped polycrystalline layer of which the dopant tends to diffuse out and cause humps.
2. Description of Related Art
Polycrystalline materials like Poly-Si or Poly-SiGe are important semiconductor materials in various semiconductor processes, and can be in-situ doped with a p-type or n-type dopant. Sometimes an excess amount of the dopant may separate and diffuse out of the polycrystalline material along the grain boundaries, causing certain problems.
Referring to FIG. 1, a poly-Si layer 110 is deposited on a substrate 100, including many grains 112 and in-situ doped with phosphorus. Out-diffusing phosphorus 120 is present over some boundaries of the grains 112. In subsequent deposition of a silicon nitride (SiN) layer 140, SiON bumps 130 are formed over the out-diffusing phosphorus 120, so that SiN bumps 150 are formed on the SiN layer 140 lowering the accuracy of a subsequent lithography process or a pattern transfer to the poly-Si layer 110.
For example, when the poly-Si layer 110 is to be patterned into separate parallel word lines using a mask consisting of a plurality of spacers previously formed on the sidewalls of patterns of the SiN hard mask layer 140 defined by a lithography process, bridging of adjacent word lines easily occurs due to the humps 150.